Ethylene Recovery From Off-Gas

ABSTRACT

A method for the recovery of ethylene from an off-gas is provided. This method includes providing an ethylene containing feed gas stream, introducing the ethylene containing gas stream into a first cryogenic separation device, thereby producing a first condensed stream and a first ethylene poor stream, and introducing the ethylene poor stream into a first feed compressor, thereby producing a pressurized first ethylene poor stream. This is repeated three times, resulting in a pressurized third ethylene poor stream, which is introduced into a pressure swing adsorber, thereby producing a high purity hydrogen stream and a PSA tail gas stream. The PSA tail gas stream is into a PSA tail gas compressor, thereby producing a pressurized PSA tail gas stream, which is then combined with an ethylene containing gas stream to produce the ethylene containing feed gas stream. The first condensed stream, the second condensed stream and the third condensed stream are combined to produce a combined condensed stream, which is introduced into an ethylene separation column thereby producing an ethylene rich stream and a methane rich stream. The methane rich stream is introduced into a methane compressor thereby producing a methane fuel stream.

BACKGROUND

Ethylene is recovered from light gas mixtures such as cracked gas fromhydrocarbon crackers which contain various concentrations of hydrogen,methane, ethane, ethylene, propane, propylene, and minor amounts ofhigher hydrocarbons, nitrogen, and other trace components. The recoveryof ethylene from crude light hydrocarbon gas mixtures is an economicallyimportant but highly energy intensive process. A method of separatingethylene which is simple, durable and cost-effective is desirable

SUMMARY

A method for the recovery of ethylene from an off-gas is provided. Thismethod includes providing an ethylene containing feed gas stream,introducing the ethylene containing gas stream into a first cryogenicseparation device, thereby producing a first condensed stream and afirst ethylene poor stream, and introducing the ethylene poor streaminto a first feed compressor, thereby producing a pressurized firstethylene poor stream. This is repeated three times, resulting in apressurized third ethylene poor stream, which is introduced into apressure swing adsorber, thereby producing a high purity hydrogen streamand a PSA tail gas stream. The PSA tail gas stream is into a PSA tailgas compressor, thereby producing a pressurized PSA tail gas stream,which is then combined with an ethylene containing gas stream to producethe ethylene containing feed gas stream. The first condensed stream, thesecond condensed stream and the third condensed stream are combined toproduce a combined condensed stream, which is introduced into anethylene separation column thereby producing an ethylene rich stream anda methane rich stream. The methane rich stream is introduced into amethane compressor thereby producing a methane fuel stream.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an embodiment of the present invention.

FIG. 2 illustrates another embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Illustrative embodiments of the invention are described below. While theinvention is susceptible to various modifications and alternative forms,specific embodiments thereof have been shown by way of example in thedrawings and are herein described in detail. It should be understood,however, that the description herein of specific embodiments is notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

As the off gas is chilled in the cold box, methane and ethylene condenseand drop out as liquid stream. The stream is chilled in several stagesto economize on the compression power of Feed Compressor.

The condensed stream is sent to an ethylene separator column, whereethylene is concentrated at the bottom and methane rich stream isremoved at the top. The column is driven by providing heat in a reboilerfrom a suitable stream within the process. An ethylene rich stream maycontain 50-99% ethylene, balance being mainly methane. This stream maybe recycled to ethylene plant for further purification or furtherpurified in a separate system.

Turning now to FIG. 1, a method for the recovery of ethylene from anoff-gas is provided. This method includes providing an ethylenecontaining feed gas stream 102, and introducing the ethylene containingfeed gas stream 102 into feed compressor 103, thereby producing apressurized ethylene containing feed stream 104. Then pressurizedethylene containing feed stream 104 is introduced into a cryogenicseparation device 105, thereby producing an ethylene rich stream 108, ahigh pressure methane rich stream 106, a low pressure methane richstream 114, and an ethylene poor stream 107.

The ethylene poor stream 107 is introduced into a pressure swingadsorber 109, thereby producing a high purity hydrogen stream 110 and aPSA tail gas stream 111. The PSA tail gas stream 111 is introduced intoa PSA tail gas compressor 112, thereby producing a pressurized PSA tailgas stream 113, which is then combined with an ethylene containing gasstream 101 to produce the ethylene containing feed gas stream 102. Thelow pressure methane rich stream 114 is introduced into a methanecompressor 115 thereby producing a methane fuel stream 116.

The ethylene containing gas stream 101 may have less than 10% ethylene.The ethylene containing gas stream 101 may have less than 5% ethylene.The ethylene containing gas stream 101 may have less than 60% methane.The ethylene containing gas stream 101 may have less than 50% hydrogen.The ethylene containing gas stream 101 may have a composition of about40% hydrogen, about 58% methane, and about 2% ethylene. The high purityhydrogen stream 110 may have a pressure of about 300 psig. The methanefuel stream 116 may have a pressure of about 75 psig.

Turning now to FIG. 2, a method for the recovery of ethylene from anoff-gas is provided. This method includes providing an ethylenecontaining feed gas stream 203, and introducing the ethylene containinggas stream 201 into a first cryogenic separation device 204, therebyproducing a first condensed stream 206 and a first ethylene poor stream205. The ethylene poor stream 205 is introduced into a first feedcompressor 207, thereby producing a pressurized first ethylene poorstream 208. The pressurized first ethylene poor stream 208 is introducedinto a second cryogenic separation device 209, thereby producing asecond condensed stream 211 and a second ethylene poor stream 210.

The second ethylene poor stream 210 is introduced into a second feedcompressor 212, thereby producing a pressurized second ethylene poorstream 213. The pressurized second ethylene poor stream 213 isintroduced into a third cryogenic separation device 214, therebyproducing a third condensed stream 216 and a third ethylene poor stream215. The third ethylene poor stream 215 is introduced into a third feedcompressor 217, thereby producing a pressurized third ethylene poorstream 218. The pressurized third ethylene poor stream 218 is introducedinto a pressure swing adsorber 219, thereby producing a high purityhydrogen stream 220 and a PSA tail gas stream 221.

The PSA tail gas stream 221 is introduced into a PSA tail gas compressor222, thereby producing a pressurized PSA tail gas stream 223, which isthen combined with an ethylene containing gas stream 201 to produce theethylene containing feed gas stream 203. The first condensed stream 206,the second condensed stream 211 and the third condensed stream 216 arecombined to produce a combined condensed stream 224. The combinedcondensed stream 224 is introduced into an ethylene separation column225 thereby producing an ethylene rich stream 226 and a methane richstream 227. The methane rich stream 227 is introduced into a methanecompressor 228 thereby producing a methane fuel stream 229.

The ethylene containing gas stream 201 may have less than 10% ethylene.The ethylene containing gas stream 201 may have less than 5% ethylene.The ethylene containing gas stream 201 may have less than 60% methane.The ethylene containing gas stream 201 may have less than 50% hydrogen.The ethylene containing gas stream 201 may have a composition of about40% hydrogen, about 58% methane, and about 2% ethylene. The high purityhydrogen stream 220 may have a pressure of about 300 psig. The methanefuel stream 229 may have a pressure of about 75 psig.

1: A method for the recovery of ethylene from an off-gas, comprising; a)providing an ethylene containing feed gas stream, b) introducing theethylene containing feed gas stream into a feed compressor, therebyproducing a pressurized ethylene containing feed stream, c) introducingsaid pressurized ethylene containing gas feed stream into a cryogenicseparation device, thereby producing an ethylene rich stream, a highpressure methane rich stream, a low pressure methane rich stream, and anethylene poor stream, d) introducing said ethylene poor stream into apressure swing adsorber, thereby producing a high purity hydrogen streamand a PSA tail gas stream, e) introducing said PSA tail gas stream intoa PSA tail gas compressor, thereby producing a pressurized PSA tail gasstream, which is then combined with an ethylene containing gas stream toproduce said ethylene containing feed gas stream, f) introducing saidlow pressure methane rich stream to a methane compressor therebyproducing a methane fuel stream. 2: The method of claim 1, wherein saidethylene containing gas stream has less than 10% ethylene. 3: The methodof claim 2, wherein said ethylene containing gas stream has less than 5%ethylene 4: The method of claim 2, wherein said ethylene containing gasstream has less than 60% methane. 5: The method of claim 2, wherein saidethylene containing gas stream has less than 50% hydrogen. 6: The methodof claim 1, wherein said ethylene containing gas stream has acomposition of about 40% hydrogen, about 58% methane, and about 2%ethylene. 7: The method of claim 1, wherein said high purity hydrogenstream has a pressure of about 300 psig. 8: The method of claim 1,wherein said methane fuel stream has a pressure of about 75 psig. 9: Amethod for the recovery of ethylene from an off-gas, comprising; a)providing an ethylene containing feed gas stream b) introducing saidethylene containing gas stream into a first cryogenic separation device,thereby producing a first condensed stream and a first ethylene poorstream, c) introducing said ethylene poor stream into a first feedcompressor, thereby producing a pressurized first ethylene poor stream,d) introducing said pressurized first ethylene poor stream into a secondcryogenic separation device, thereby producing a second condensed streamand a second ethylene poor stream, e) introducing said second ethylenepoor stream into a second feed compressor, thereby producing apressurized second ethylene poor stream, f) introducing said pressurizedsecond ethylene poor stream into a third cryogenic separation device,thereby producing a third condensed stream and a third ethylene poorstream, g) introducing said third ethylene poor stream into a third feedcompressor, thereby producing a pressurized third ethylene poor stream,h) introducing said pressurized third ethylene poor stream into apressure swing adsorber, thereby producing a high purity hydrogen stream(120) and a PSA tail gas stream, i) introducing said PSA tail gas streaminto a PSA tail gas compressor, thereby producing a pressurized PSA tailgas stream, which is then combined with an ethylene containing gasstream to produce said ethylene containing feed gas stream, j) combiningsaid first condensed stream, said second condensed stream and said thirdcondensed stream to produce a combined condensed stream, k) introducingsaid combined condensed stream into an ethylene separation columnthereby producing an ethylene rich stream and a methane rich stream, andl) introducing said methane rich stream to a methane compressor therebyproducing a methane fuel stream. 10: The method of claim 9, wherein saidethylene containing gas stream (101) has less than 10% ethylene. 11: Themethod of claim 10, wherein said ethylene containing gas stream has lessthan 5% ethylene 12: The method of claim 10, wherein said ethylenecontaining gas stream has less than 60% methane. 13: The method of claim10, wherein said ethylene containing gas stream has less than 50%hydrogen. 14: The method of claim 9, wherein said ethylene containinggas stream has a composition of about 40% hydrogen, about 58% methane,and about 2% ethylene. 15: The method of claim 9, wherein said highpurity hydrogen stream has a pressure of about 300 psig. 16: The methodof claim 9, wherein said methane fuel stream has a pressure of about 75psig.